Femtosecond time-resolved photoelectron spectroscopy with a vacuum-ultraviolet photon source based on laser high-order harmonic generation

Abstract

A laser-based tabletop approach to femtosecond time-resolved photoelectron spectroscopy with photons in the vacuum-ultraviolet (VUV) energy range is described. The femtosecond VUV pulses are produced by high-order harmonic generation (HHG) of an amplified femtosecond Ti:sapphire laser system. Two generations of the same set up and results from photoelectron spectroscopy in the gas phase are discussed. In both generations a toroidal grating monochromator was used to select one harmonic in the photon energy range of 20-30 eV. The first generation of the set up was used to perform photoelectron spectroscopy in the gas phase to determine the bandwidth of the source. We find that our HHG source has a bandwidth of 140±40 meV. The second and current generation is optimized for femtosecond pump-probe photoelectron spectroscopy with high flux and a small spot size at the sample of the femtosecond probe pulses. The VUV radiation is focused into the interaction region with a toroidal mirror to a spot smaller than 100 x 100 μm 2 and the flux amounts to 10 10 photons/s at the sample at a repetition rate of 1 kHz. The duration of the monochromatized VUV pulses is determined to be 120 femtoseconds resulting in an overall pump-probe time resolution of 135±5 fs. We show how this set up can be used to map the transient valence electronic structure in molecular dissociation.3

A Beamline for Time-Resolved Extreme Ultraviolet and Soft X-Ray Spectroscopy

“…Indeed, Labs is reasonably close to 1/3 of the cell length for the dominant harmonics. Moreover, we find our results to be in line with those of Takahashi et al [52] and similar to the parameters used by Wernet et al [33] or He et al [54].…”

“…Pioneering time resolved studies using EUV carries an intensity dependent phase, making phase matching much more demanding [24][25][26]. Popular schemes to make use of some phase matching include gas jets [27][28][29], gas cells [16,[30][31][32][33][34] multiple gas cells in series [35] hollow core capillaries [36] and plasma plumes [37].…”

Monochromatization of femtosecond XUV light pulses with the use of reflection zone plates

“…The latter condition imposes an upper limit onto the intensity value. In previous cross-correlation experiments with the use of Ti:sapphire laser pulses of femtosecond duration, the peak IR intensity was restricted to a value below 10 13 W/cm 2 [15,40]. At such intensities, the process of above-threshold ionization (ATI) of Ar in the IR field has a negligible yield at high electron kinetic energies and, thus, an overlap of the ATI spectrum with the cross-correlation spectrum is avoided as well.…”

A high-order harmonic generation apparatus for time- and angle-resolved photoelectron spectroscopy

“…The main differences between the setups are the repetition rate of the laser and the photon-energy selection, which is the primary focus of this paper. Currently there are several different approaches for wavelength selection, most common are single grating 19,23,32 and double grating 29,33,34 monochromators. Generally single grating arrangements have a higher transmission while double grating arrangements deliver shorter pulses by compensating the tilt of the wave front incurred by diffraction from a single grating.…”

“…Photoelectrons generated at these photon energies have mean free paths in the material of only a fewÅngstrom, so this technique is inherently surface sensitive and probes the first few sub-surface layers to yield information on the bulk band-structure. Since the native bandwidth of even a single harmonic generated by a 40 fs near infrared laser pulse is on the order of 0.25 − 0.5 eV, 32 it is desirable to select a fraction of the bandwidth. To this end we have designed and built a monochromator beamline delivering a bandwidth as small as 90 meV at 35.6 eV, while maintaining an overall time resolution better than 150 fs and sufficient photon flux for reasonable measurement times.…”